Regulatory T cells (Treg) are immune suppressive T cells that are critical to control self-tolerance and immune homeostasis. Intact function of Foxp3, an X-linked transcription factor specifically expressed in Treg cells, is essential to maintain normal function of Treg cells because perturbation of Foxp3, however slight, often leads to aberrant function of Treg cells and the development of inflammatory diseases and autoimmunity. Dysregulated Treg cells may lose immune suppressive function and also gain effector function. Therefore, elucidating how Foxp3 expression and Treg function are controlled is of great importance for the understanding of immune regulation and tolerance. Increasing evidence supports the notion that mechanisms exist in Treg cells to sense the environmental cues to adjust their function during different types of immune response through the functions of transcription factors. Yet, our knowledge on transcriptional control of Treg function is still far from complete. For years, we have been interested in identifying transcription factors important for Treg function. Recently, we found that Gata3 is preferentially expressed by Foxp3+ Treg cells, and that GATA3 expression in Treg cells can be modulated by cytokine milieu, suggesting that GATA3 is involved in controlling Treg cell functions. Gata3 is a transcription factor initialy identified as the master regulator for Th2 differentiation. GATA3 is also required for T cell development and NK cell function. Therefore, Gata3 plays critical roles in controlling diverse functions of different types of immune cells. Nevertheless, whether and how Gata3 regulates Treg functions remain to be revealed. We have investigated Gata3 function in Treg cells by deleting the Gata3 gene specifically in Foxp3+ cells and found that Gata3 deficiency in Treg cells led to the development of inflammatory disorders in mice. Treg cells lacking Gata3 displayed reduced Foxp3 expression, defective immune suppression, impaired peripheral maintenance and aberrant Th differentiation. These findings demonstrated that Gata3 is essential for Foxp3 expression and Treg function. Intrigued by these findings, here we propose to study this novel function of GATA3 in Treg cells. The overarching goal for this proposed study is to elucidate the underlying mechanism(s) through which Gata3 modulates Treg function and related immune response. To reach this goal, and specifically, to understand how Gata3 controls Treg function through Foxp3 dependent and independent mechanisms, by generating various genetic modified mice strains and by combining molecular, biochemical and genetic approaches, we propose to accomplish following research aims: Aim 1. Study how Gata3 controls Foxp3 expression. Aim 2. Investigate the mechanisms through which Gata3 controls Treg homeostasis. Aim 3. Determine the function of Gata3 in controlling Th differentiation of Treg cells. Gata3 is a transcription factor previously found to be critical for Th2 differentiatio, T cell development and NK cell function. Our recent findings revealed a novel essential role of GATA3 in controlling Treg function. This proposed study will uncover the mechanism(s) underlying the fundamental role of GATA3 in Treg cell physiology, Foxp3 expression and immune regulation. The success of this study will enhance the knowledge of immune regulation, will further the understanding of etiology of autoimmune and inflammatory disease, and may guide the development of effective therapies against immune diseases.